You might have noticed Picard/Stewart does the same thing. It's kind of a prereq to professional acting. Maybe if he had mumbled through it all?
Heh. I've always thought Clu Gulager was ahead of his time because he did a lot of eccentric roles where he spoke under his breath a lot, acted 'out' instead of to the actor. Would have some business going on. I found him refreshing and realistic....but last night I was watching him on the MST3K version of San Francisco International, and he would say a few lines and the Bots would go "Mrrrrbullllessssm mummmblessss mrrrummmblesss"